1. Field of the Invention
The present invention relates to electric power distribution systems, and more particularly to a method and apparatus for incorporating thermal memory effects into a Solid State Power Controller (SSPC), to achieve coordination between a fuse and the SSPC.
2. Description of the Related Art
Solid State Power Controllers (SSPCs) are poised to become important components in electric power distribution systems for commercial aircraft applications. Advantages of SSPC technology include: light weight and small size of SSPC devices, reduced maintenance requirements, and increased reliability. Hence, the SSPC technology represents an attractive alternative to conventional distribution systems including electro-mechanical relays and circuit breakers.
However, the possibility of fail-short events for the SSPCs presents serious safety considerations. These safety considerations are a critical element in the certification of the SSPC technology for commercial aircraft. In order to meet the safety and reliability requirements enforced by certification authorities, an additional protection mechanism is often associated with the SSPC. The interaction between the SSPC and the additional protection mechanism has added further complexity to the use of SSPC technology in commercial aircraft.
One known method that incorporates a thermal memory into an SSPC is described in U.S. Pat. No. 5,723,915 titled “Solid State Power Controller”, by T. R. Maher et al. With the technique described in this patent, a thermal memory feature is used with an SSPC, to mimic the performance characteristics of traditional circuit breakers. This technique, however, does not address the safety concern of the fail-short mode of the MOSFET in the SSPC. Moreover, the circuit implementation of this technique requires resistance values that are not recommended for aerospace applications, and capacitance values that can cause high leakage and high variations in trip timing.
Another known method that associates an additional protection mechanism with an SSPC is described in U.S. Pat. No. 5,287,078 titled “Safety Fuse Apparatus for Solid State Power Controllers”, by E. K. Larson. With the technique described in this patent, a U-shaped, metal alloy based safety device is associated with an SSPC. The safety device has performance characteristics that conform to selected SSPC time-current curves, to avoid interference with normal operation of the SSPC. The use of this new U-shaped metal alloy based safety device, however, can compromise the optimization of wire selection and faces problems with the certification of the SSPC technology for commercial aircraft, due to lack of field experience.
Disclosed embodiments of this application address these and other issues by implementing SSPC trip mechanisms with thermal memory effect, to coordinate between an SSPC trip curve and the characteristics of an additional protection mechanism, such as a fuse. Embodiments of the present invention divide a thermal memory effect into an initial thermal memory and a thermal memory due to over current, to achieve practical, semi-digital circuit implementations of the thermal memory effect. In one embodiment, a capacitor is charged multiple times during an over current condition, to produce a count that represents the temperature in a wire subjected to the over current. Temperature variation in the wire during the over current condition is represented with a discharge capacitor, a resistor, and a voltage associated to the trip count.